1. Field of the Invention
The present invention relates to an AC generator for a vehicle which supplies three-phase AC power driven by an engine.
2. Description of the Related Art
It is well-known that in a conventional AC generator which has a field rotor with a Lundell-type pole core and a field coil, permanent magnets are disposed on the circumference of the Lundell-type pole core between the adjacent claw pole pieces thereof, for example, as disclosed in U.S. Pat. No. 4,959,577. The permanent magnets is polarized in a direction opposite to the pole of the leakage flux between the claw pole pieces to reduce the leakage flux and increase the effective flux, thereby increasing the output power of the generator.
However, such a generator has the following problems.
First, because the permanent magnets are disposed opposite ends of the pole core, the effect of the permanent magnet is obtained in the direction of the rotation. As a result, the distribution of the magnetic flux of the pole piece forms into a non-sinusoidal wave which includes higher harmonics as shown in FIG. 23. Therefore, higher harmonics in the current induced in the stator winding is generated to cause an electric noise and a magnetic noise generated by the higher harmonics of the current, causing a problem of such a rotor with the permanent magnets.
Second, in the above rotor with the permanent magnets, demagnetizing force of the armature reaction as well as the magnetomotive force of the field coil reduces the magnetic coercive force of the permanent magnets.
Third, because the permanent magnets are disposed near the outer circumference of the rotor having the Lundell-type pole core with the permanent magnets between the claw pole pieces, they are subject to change in the magnetomotive force of the stator winding as shown in FIGS. 24A-24D. As a result, the permanent magnets are demagnetized, reducing the output power of the generator. In addition, the pole pieces are heated by the magnetic disturbance, resulting in reduction of the output power and in demagnetizing permanent magnet made of a high performance magnetic material such as rare earth metal.
Forth, because a portion of the flux is supplied to the stator core by the permanent magnets even when the field coil is not excited and any load is connected, the output voltage exceeds the battery voltage as shown in FIG. 25 if the magnetic flux is large in a high rotational speed range of the generator, resulting in overcharging of the battery.
In general, the electromagnetic noise is mainly caused by the higher harmonics included in the distribution of magnetomotive force. Therefore, it is necessary to reduce the higher harmonics of the magnetomotive force.
It is necessary to reduce the armature reaction in order to prevent the demagnetization of the permanent magnets. It is also necessary to reduce the change in the distribution of the magnetomotive force in order to reduce the magnetic disturbance.
In order to solve the problem of the over-charging of the battery, it is necessary to reduce the voltage including higher harmonics generated by the flux of permanent magnet at the no-load operation.